The field of this invention is radio communications systems, and more particularly, systems for radio direction finding.
One form of prior art system utilizes a radio frequency (RF) transmitter and receiver located at opposite ends of a radio communication path, with a continuously rotating directional antenna located at either the transmitting or receiving end of the path, and an omni-directional antenna located at the other end. A reference signal is provided at the receiving end of the path to identify a particular point (such as North) in the cycle of the rotating antenna. The rotating antenna causes a periodic modulation of the RF signal (AM, or FM, or a combination of both), and this modulation is detected by the receiver. The demodulated output signal provided by the receiver is periodic at the rotational frequency of the antenna, with the phase of that output signal relative to that of the reference signal being a measure of the relative bearing of the receiver and transmitter locations. In known prior art systems, the rotating antenna may be physically rotated (as by a motor), or alternatively, with a multiple element configuration, the elements may remain fixed with the pattern rotated by an electronic modulating arrangement.
One problem encountered by the prior art radio direction finding systems is bearing indication error due to phase shift introduced by the receiver. The usual remedy in such systems is a phase adjustment provided in the receiver to offset the characteristic phase delay for the particular receiver. In general, the characteristic phase shift varies with the individual receivers, and thus each receiver must be separately adjusted. When the antenna rotational frequency is low, (such as 30 Hz of the so-called Omnirange systems) relatively easily compensated delays in the audio circuits are generally the only consideration. However, if the antenna rotational frequency is high, and approaches the bandwidth of the receiver, compensation poses a much more difficult problem, particularly if the transmitter center-frequency deviates somewhat from its nominal value. Under such conditions, the time delay (caused primarily by characteristics of the IF filters) suffered by the intelligence-bearing side bands of the RF signal generally varies with various system parameters such as receiver tuning. These variations can easily be substantial compared with the rotational period of the antenna, thereby leading to correspondingly high error in the bearing indication. Since the frequency-determining components (e.g. crystals) of both the transmitter and receiver are necessarily provided with certain tolerances with regard to center-frequency, even in the most rigidly controlled communication systems, this source of error may become substantial.
Thus, in the prior art radio direction finding systems, variations in a number of factors can cause substantial errors due to receiver introduced phase shift, with such errors not always being easily predicted or corrected. One such factor is the variation in tuning of either the transmitter or receiver from the exact "center-frequency" (the variation may be dependent upon temperature, power supply, or simply arise because two different transmitters or receivers happen to be tuned within their specified tolerances but slightly differently). A second factor contributing such error is variation in the rotational frequency of the antenna (such as might be due to temperature, or power supply variation).
A third source of error lies in the tuning and adjustment of the receiver audio filters or other circuits, which may be affected by temperature, power supply, aging so that frequent re-calibration is required.
It is an object of the present invention to provide a radio direction finding system which minimizes the effect of receiver-introduced phase shift.
It is another object of the present invention to provide a radio system for producing a bearing indication which is substantially free from error due to receiver-introduced phase shift.